Semiconductor diodes are well known in the art. Generally speaking, a diode functions as an electrical valve allowing current to flow in a forward direction from its anode to its cathode, with a known voltage drop associated with its p-n junction. When current is applied in the opposite direction, however, the diode acts as an open circuit, preventing current flow in that direction. In many applications, such as power applications, it is desirable to reduce or minimize the forward voltage drop of the diode to reduce or minimize its power dissipation. A diode having a 700 mV forward voltage drop, for example, will dissipate 7 watts of power when it conducts 10 amps of current, which is unsuitable for many applications.
Accordingly, this power dissipation problem has motivated circuit designers to design switching circuits having as small a forward voltage drop as possible in an attempt to produce an “ideal diode” circuit (i.e., one which turns ON substantially instantaneously with substantially no forward voltage drop).
Such circuits generally include a semiconductor switch as a pass element (such as MOSFETs) that has a small forward voltage drop (typically under 100 mV) and thus significantly reduce power dissipation. One drawback of these circuits is the relatively long turn ON time associated with the switches as compared to conventional diodes. For example, a switch-based ideal diode circuit may have a turn ON time of 50 microseconds whereas a conventional diode may turn ON in picoseconds. This lag in turn ON time may be unsuitable for applications that require strict load regulation or fast response times.
One example of a semiconductor based ideal diode circuit is the LTC4355 produced by Linear Technology Corporation of Milpitas Calif., the assignee of this patent application. The LTC4355 employs a charge pump circuit to generate a voltage of sufficient magnitude to drive the gate of a MOSFET switch. The low output current of the charge pump, however, causes a relatively slow turn ON of the MOSFET, which is not optimal for low voltage applications.
Another ideal diode circuit is the LT4351 device, produced by Linear Technology, which relies on an external supply or a boost regulator for the gate drive. The use of a boost regulator requires an inductor, which occupies significant board area and generates noise due to its relatively large and fast switching currents, which may cause gate node chatter, disturbing load voltage.
Similarly, the MAX5079 device from Maxim Integrated Products uses a charge pump and an external flying capacitor to deliver 2 mA (for a 100 nF flying capacitor) of switch pull up current to drive a switch gate, providing a 25 microsecond switching time, which is too slow for some ideal diode applications. With this approach, increasing the size of the flying capacitor to improve drive current tends to cause excessive ripple at the input, degrading circuit performance. The TPS2410 ideal diode device produced by Texas Instruments suffers from similar drawbacks.